Caption: Melatonin. Light micrograph in polarised light of crystals of the hormone melatonin. Melatonin is secreted by the pineal gland in the brain that controls the body's biological rhythm. It is secreted at night and induces sleep. Pills containing melatonin can be taken to prevent jetlag. In middle age, melatonin secretion drops off and may be responsible for aging symptoms such as insomnia and irritability. Horizontal object-size 1.2mm.